When do you use y and when do you use en in French?

Y replaces a + a thing or a place: 'je pense a mon travail' becomes 'j'y pense' (I think about it), 'je vais a Paris' becomes 'j'y vais' (I'm going there). En replaces de + a thing, and it also covers partitives (du, de la, des) and quantities: 'je veux du cafe' becomes 'j'en veux' (I want some), 'j'ai trois livres' becomes 'j'en ai trois' (I have three of them). The shortcut: a triggers y, de triggers en. Neither is used for people - for a person you use lui / leur or a stressed pronoun (je pense a lui).

Why does manger become manges-en in the imperative?

In the affirmative imperative the pronoun attaches to the verb with a hyphen, and the tu form of -er verbs gets its silent 's' back before y and en for ease of pronunciation. 'Mange!' becomes 'manges-en!' (eat some) and 'va!' becomes 'vas-y!' (go on / go there). The 's' is purely euphonic - it stops two vowel sounds colliding (mange-en would be awkward to say). It only appears before y and en, not before other pronouns: 'mange-le' keeps no 's'.

Can y and en replace people in French?

No. Y and en are for things, places and quantities, not people. 'Je pense a mon frere' (I think about my brother) cannot become 'j'y pense' - it becomes 'je pense a lui' with a stressed pronoun. Likewise 'j'ai besoin de toi' (I need you) stays as it is, you do not say 'j'en ai besoin' about a person. With quantities, though, en is fine even when counting people: 'j'ai deux freres' becomes 'j'en ai deux' (I have two of them), because here en is standing in for the quantity, not the person.

French Pronouns Y and EN

Y and en are the cleanup crew. Every time a sentence ends in a + a thing, de + a thing, or a quantity, one of these two pronouns can swallow the lot. They sit before the verb like ordinary object pronouns, and the only real work is matching the right pronoun to the right preposition: a triggers y, de triggers en.

What each pronoun replaces

Pronoun	Replaces	Example
y	a + thing or place	j'y vais (I'm going there)
en	de + thing	j'en parle (I talk about it)
en	partitive (du / de la / des)	j'en veux (I want some)
en	a quantity / number	j'en ai trois (I have three of them)

Y: replacing a + a thing or a place

Y stands in for a plus a thing, or for a place introduced by any location preposition (a, dans, sur, chez).

Je vais a Paris. -> J'y vais. (I'm going there.)
Je pense a mon travail. -> J'y pense. (I think about it.)
Il repond a la question. -> Il y repond. (He answers it.)
Les cles sont sur la table. -> Les cles y sont. (The keys are there.)

The place sense is the easy one ("there"). The a + abstract-thing sense is the one learners forget: penser a quelque chose, repondre a quelque chose, s'interesser a quelque chose all collapse to y.

Y is never used for a person. Je pense a ma soeur does not become j'y pense; it becomes je pense a elle with a stressed pronoun.

En: replacing de + a thing, partitives and quantities

En does three related jobs, all anchored to de.

First, plain de + a thing, after verbs that take de:

Je parle de mon voyage. -> J'en parle. (I talk about it.)
Elle a besoin d'argent. -> Elle en a besoin. (She needs some.)
Je viens de la gare. -> J'en viens. (I'm coming from there.)

Second, the partitive articles du / de la / de l' / des ("some / any"):

Tu veux du cafe? -> Tu en veux? (Do you want some?)
Je mange des legumes. -> J'en mange. (I eat some.)

Third, quantities and numbers. Here the number or quantity word stays at the end while en marks what is being counted:

J'ai trois livres. -> J'en ai trois. (I have three of them.)
Elle achete beaucoup de pain. -> Elle en achete beaucoup. (She buys a lot of it.)
Il n'y a pas de lait. -> Il n'y en a pas. (There isn't any.)

Position: before the verb, or before the infinitive

Like the other object pronouns, y and en go before the conjugated verb, and before the auxiliary in the passe compose.

J'y pense souvent. (I think about it often.)
Nous en avons parle hier. (We talked about it yesterday.)

With an infinitive after a conjugated verb, they attach to the infinitive:

Je vais y aller demain. (I'm going to go there tomorrow.)
Tu peux en prendre. (You can take some.)

The imperative: attachment and the euphonic s

In the affirmative imperative, y and en attach to the verb with a hyphen. The tu form of -er verbs gets its silent -s back before y and en, purely so the words run together cleanly:

Va! -> Vas-y! (Go on. / Go there.)
Mange! -> Manges-en! (Eat some.)
Prends-en! (Take some.) - prendre is not an -er verb, so no extra s, but the tu form already ends in -s

In the negative imperative, y and en return to their normal preverbal slot and the extra s disappears:

N'y va pas. (Don't go there.)
N'en prends pas. (Don't take any.)

Order when stacked with other pronouns

When pronouns pile up, y and en come last, and en is the very last of all. The full sequence:

1st	2nd	3rd	4th	5th
me / te	le / la	lui	y	en
nous / vous	les	leur
se

Il m'en donne. (He gives me some.)
Je l'y emmene. (I'm taking him there.)
Donne-m'en. (Give me some.) - imperative; me becomes m' before en
Il y en a. (There is / are some.) - y and en together, en last

The pairing y en in il y en a ("there is some") is worth memorising as a fixed unit; it turns up constantly.

Worked examples

Tu vas a la banque? - Oui, j'y vais maintenant. (Are you going to the bank? Yes, I'm going there now.)
Elle s'interesse a la politique? - Elle s'y interesse beaucoup. (Is she interested in politics? She's very interested in it.)
Combien de freres as-tu? - J'en ai deux. (How many brothers do you have? I have two.)
Tu as parle de ton probleme? - Oui, j'en ai parle au prof. (Did you talk about your problem? Yes, I talked about it to the teacher.)
Reprends du gateau! - Non merci, j'en ai deja mange. (Have some more cake. No thanks, I've already had some.)
On y va? (Shall we go? - literally "do we go there?", a set phrase.)

Common mistakes English speakers make

Using y or en for a person: j'y pense about your sister is wrong, it's je pense a elle. Matching the wrong preposition - reaching for y when the verb takes de, or en when it takes a; the fix is to learn each verb with its preposition (penser a, parler de). Dropping the quantity word in the en construction: "I have three" is j'en ai trois, not just j'en ai - the number stays. And forgetting the euphonic s in the imperative: it's vas-y and manges-en, not va-y or mange-en.